As it is known, bicycle gear-shifts are generally comprised of a group of sprockets of different diameters, mounted integral in rotation with each other. A drive chain engages a sprocket from the group according to the desired transmission ratio. Each sprocket has a plurality of teeth which follow one another at a set distance or pitch from each other. The pitch is the same on the various sprockets of the same group and corresponds to the pitch of the chain.
It should be noted that on a bicycle the motion transmission always takes place in the same direction, i.e. the chain always moves in the same direction (at least when it transmits power), and therefore the sprockets always rotate in the same direction, that is the teeth advancement direction. With reference to such a direction of rotation, each tooth has a lead side (the one facing forwards with respect to the direction of rotation) and a following side (the one facing backwards). In the sprockets of a front gear-shift group, the lead side of the tooth is the pressure side, that is the one which exerts the traction force on the chain; in the sprockets of a rear gear-shift group, the pressure side is, by contrast, the following side, upon which the traction force exerted by the chain acts. Each tooth also has an extension in the circumferential direction, or width, an extension in the radial direction, or height, and an extension in the axial direction, or thickness.
The displacement of the engagement of the chain from one sprocket to another is known as a gear-change. It is usually obtained by displacing the chain transversally with respect to its own longitudinal axis (that is axially with respect to the crown of teeth), until it is disengaged from one sprocket and engaged with the adjacent sprocket. Such a displacement action is usually obtained by chain-guide devices, mounted immediately upstream of the sprockets with reference to the motion direction of the chain. Devices of different types exist, both for front and rear gear-shifts.
If the gear-change is from a larger diameter sprocket to a smaller one, it is said to be a “downwards gear-change”; if vice-versa from a smaller to a larger diameter, it is said to be an “upwards gear-change.” It should be noted that on the front gear-shift a downwards gear-change involves a reduction in the transmission ratio, whereas on the rear gear-shift a downwards gear-change involves an increase in the transmission ratio.
The gear-change, both upwards and downwards, is in any case a delicate operation, since it takes place with the chain taut and since the chain has a very limited deformability in the transversal direction (with respect to its own longitudinal axis); the delicateness is usually greater on the front gear-shift, where the differences in diameter between adjacent sprockets are greater.
In particular, during a downwards gear-change, the chain must be disengaged from the larger sprocket and deviated towards a smaller one. Whilst it is not usually difficult to make the chain reach the small sprocket (the tension of the chain helps this), it is, however, difficult to disengage the chain from the larger sprocket. Since the chain portion to be deviated is the taut one, a substantial transversal thrusting stress is needed to be able to disengage the chain from the larger sprocket. If the necessary stress is not applied, the gear-change is denied, that is it does not take place.
Also, sometimes on the front gear-shift group it may happen that during the gear-change the chain engages with the small sprocket without however managing to disengage from the large sprocket. This can happen if, by chance, the distance (in the deviated direction which the chain takes up during the gear-change) between the last engaged tooth of the large sprocket and the first engaged tooth of the small sprocket is equal to a whole multiple of the pitch. If this happens, there is not a simple denial of the gear-change, but rather the so-called “chain eddy”, i.e. the chain gets tangled on the two sprockets and the gear-shift is clearly blocked.